Heat exchangers used in air conditioners, hot water heaters, and the like, are provided with a heat transfer pipe, wherein a fluid such as water flows, that exchanges heat due to the temperature differential between the pipe interior and exterior. Furthermore, to improve the heat transfer performance of the heat transfer pipe, it is known to use a grooved pipe, wherein grooves are formed on the pipe inner surface. In addition, a technology has also been proposed that improves heat transfer performance by providing projections on the inner surface of the heat transfer pipe.
Providing projections inside the heat transfer pipe in this manner increases the heat transfer surface area of the heat transfer pipe and agitates the fluid, thereby increasing the coefficient of heat transfer of the heat transfer surface and improving the heat transfer performance. However, if projections are provided inside the heat transfer pipe, then the projections increase the pipe coefficient of friction and raise the pressure loss of the flow inside the pipe. Therefore, a technology has been proposed (Japanese Examined Patent Application No. H06-70556) that provides projections 0.45-0.6 mm in height inside the heat transfer pipe, thereby suppressing the pressure loss while promoting the transfer of heat with the refrigerant.